As is publicly known, a variable valve timing device, which is a device to be applied to an internal combustion engine mounted on a vehicle, has been put to a practical use. The variable valve timing device varies the rotational phase of a camshaft, more specifically the rotational phase of the camshaft relative to a crankshaft, which is an engine output shaft, to set the valve timing of engine valves (intake and exhaust valves) variably. A hydraulic variable valve timing device that operates based on a hydraulic pressure as disclosed in each of Patent Documents 1 and 2 is known as such a variable valve timing device.
With reference to FIG. 3, a configuration of the variable valve timing device as disclosed in each of Patent Documents 1 and 2 will be described.
As shown in FIG. 3, a vane rotor 2 provided with a plurality of (in FIG. 3, three) vanes 3, which protrude outward in a radial direction, is fixed to a camshaft 1 such that the vane rotor 2 and the camshaft 1 rotate integrally with each other. A substantially annular housing 4 is arranged on an outer periphery of the vane rotor 2 to be rotational relative to the vane rotor 2. A cam sprocket 5, which is operably connected through a chain to a crankshaft of an internal combustion engine, is fixed to the housing 4 so that the cam sprocket 5 and the housing 4 rotate integrally with each other. Recesses 6, the number of which is the same as that of the vanes 3, are formed in an inner periphery of the housing 4. Each vane 3 is accommodated in one of the recesses 6. Each recess 6 in the housing 4 is partitioned into two oil chambers, namely a retard chamber 7 and an advance chamber 8 by the vane 3 accommodated therein. The retard chamber 7 is located on the leading side in the rotational direction of the camshaft with respect to the vane 3. In contrast, the advance chamber 8 is located on the trailing side in the rotational direction of the camshaft with respect to the vane 3.
The variable valve timing device also includes a lock mechanism, which locks the vane rotor 2 and the housing 4 such that the vane rotor 2 and the housing 4 rotate integrally with each other. The lock mechanism has a lock pin 10 that slides in a pin hole 9 formed in one of the vanes 3 of the vane rotor 2, and a lock hole 11 formed in the cam sprocket 5. The lock pin 10 can be received in the lock hole 11. The lock pin 10 is urged in a direction in which the lock pin 10 is received in the lock hole 11 by a spring 12. The lock pin 10 is arranged at a position in a rotational range of the vane rotor 2 relative to the housing 4 in which the vane rotor 2 is relatively rotated in the direction opposite to the rotational direction of the camshaft at most (hereinafter, referred to as a most retarded position), and the lock pin 10 is received in the lock hole 11.
In contrast, a hydraulic circuit, which supplies and discharges a hydraulic pressure for operating the variable valve timing device, is provided in the variable valve timing device. In the hydraulic circuit, an oil pump 14 pressurizes oil pumped from an oil pan 13, and discharges the oil to a main gallery 21 formed in a cylinder block. A hydraulic sensor 22, which detects a hydraulic pressure in the main gallery 21, is installed on the main gallery 21.
The main gallery 21 is connected through a supply oil passage 15 to an oil control valve (hereinafter, referred to as OCV 16). The OCV 16 is configured as an electromagnetic valve controlled by an electronic control unit for engine control (hereafter, referred to as ECU 20). In addition to the above-described supply oil passage 15, a drain oil passage 17 for returning the oil to the oil pan 13, a retard oil passage 18 connected to each retard chamber 7, and an advance oil passage 19 connected to each advance chamber 8 are connected to the OCV 16. The OCV 16 is configured to supply and discharge the oil to and from the retard chamber 7 and the advance chamber 8 by switching the connection of any one of the supply oil passage 15 and the drain oil passage 17 to each of the retard oil passage 18 and the advance oil passage 19.
The hydraulic pressure supplied to the retard chamber 7 and the advance chamber 8 also acts on the lock pin 10. The hydraulic pressure acts in the direction in which the lock pin is drawn out of the lock hole 11 against the urging force of the spring 12.
Next, an operation of such a variable valve timing device will be described.
When the ECU 20 as a control section commands the OCV 16 such that the supply oil passage 15 and the advance oil passage 19 are connected to each other, and the drain oil passage 17 and the retard oil passage 18 are connected to each other, the hydraulic pressure in the advance chamber 8 is increased, and the hydraulic pressure in the retard chamber 7 is reduced. Accordingly, force in the rotational direction of the camshaft (hereinafter, referred to as an advance direction) acts on the vane 3 due to the difference between the hydraulic pressures in the retard and advance oil chambers so that the vane rotor 2 rotates in the advance direction relative to the housing 4. As a result, the rotational phase of the camshaft 1 fixed to the vane rotor 2 in an integrally rotational manner is advanced with respect to the rotational phase of the cam sprocket 5 so that the valve timing of an engine valve driven to be opened and closed by the camshaft 1 is advanced.
When the ECU 20 commands the OCV 16 to connect the supply oil passage 15 and the retard oil passage 18 to each other, and connect the drain oil passage 17 and the advance oil passage 19 to each other, the hydraulic pressure in the retard chamber 7 is increased, and the hydraulic pressure in the advance chamber 8 is reduced. Accordingly, force in the direction opposite to the rotational direction of the camshaft (hereinafter, referred to as a retard direction) acts on the vane 3 due to the difference between the hydraulic pressures in the retard chamber 7 and the advance chamber 8 so that the vane rotor 2 rotates in the retard direction relative to the housing 4. As a result, the rotational phase of the camshaft 1 fixed to the vane rotor 2 in an integrally rotational manner is retarded with respect to the rotational phase of the cam sprocket 5 so that the valve timing of the engine valve driven to be opened and closed by the camshaft 1 is retarded.
In contrast, when the ECU 20 commands the OCV 16 to stop the supply and the discharge of the oil to and from both the retard oil passage 18 and the advance oil passage 19, the vane rotor 2 stops at a position at which the hydraulic pressure in the retard chamber 7 and the hydraulic pressure in the advance chamber 8 are balanced with each other. Accordingly, the valve timing of the engine valve at this time is maintained constant.
When the engine is started, the vane rotor 2 is arranged at the most retarded position. The vane rotor 2 is in the state where the lock pin 10 is received in the lock hole 11 so that the vane rotor 2 is locked at the most retarded position in the rotational manner integrally with the housing 4.
After the engine is started, when the engine rotation speed is increased so that the discharge pressure of the oil pump 14 is sufficiently increased, the ECU 20 commands the OCV 16 to connect the supply oil passage 15 and the advance oil passage 19 to each other to supply the hydraulic pressure to the advance chamber 8. The hydraulic pressure supplied to the advance chamber 8 at this time also acts on the lock pin 10 so that the lock pin 10 exits the lock hole 11 by the hydraulic pressure. As a result, the lock of the lock mechanism is released so that the relative rotation of the vane rotor 2 and the housing 4 is permitted. Accordingly, the initial command of the supply of the hydraulic pressure to the advance chamber 8 by the ECU 20 after the engine is started is equivalent to a command of applying the lock-release hydraulic pressure for releasing the lock of the lock mechanism.